Electromagnetic lifts are used in a variety of industrial and commercial applications, such as those that require lifting and transporting large metallic objects. These electromagnets are typically powered by a DC generator. A motor drives the DC generator, which produces a magnetizing current to the coils of the electromagnetic device. The current induces a strong magnetic field on the face of the electromagnetic device, which can be used to lift and transport metallic or magnetic materials. In work environments that require the lifting and transport of heavy loads, electromagnetic lifts may require a substantially large current that is sufficient to induce a magnetic field strong enough to provide the required lift capacity.
Electromagnetic lifts may be employed in harsh environments (e.g., scrap yards, shipyards, or waste facilities) and/or may be subject to prolonged exposure to the elements. Consequently, electromagnetic lifts, and components and systems associated therewith, may be particularly vulnerable to damage and wear. For example, in material handling machines used in scrap yards, current produced by an on-board generator may be carried by an electrical cable along the boom of the material handler to the electromagnet attached to the end of the boom. Should the electromagnetic cable become damaged, cut, or shorted, a resulting current overload condition may potentially harm the generator set, the electromagnet, or the electronic control equipment.
Additionally, electromagnetic lifts typically include one or more operator-controlled switches that interrupt the current flow through the electromagnet, which causes the magnetic field to weaken quickly, thereby releasing objects held by the magnetic field. Due to the high current flow required to energize the electromagnet, operation of these switches may produce electrical arcing that can lead to premature wear of the switch contacts and/or conductors associated with the electromagnet, which, over time, may result in current overload conditions within the electromagnetic system.
One method used to mitigate the effects of current overload conditions in electromagnetic lift systems involves using a fuse in the electromagnetic circuit. When the current level in the electromagnetic circuit exceeds the current rating of the fuse, the fuse breaks the circuit, thereby preventing the flow of current within the circuit. While this may be useful in disabling the electromagnetic drive circuit when a minimum current threshold is exceeded, it may be lead to unnecessary equipment downtime and increased repair costs.
Moreover, it may be advantageous in certain situations to allow temporary operation of the electromagnetic lift at peak levels that may exceed the current rating of the fuse, for very short time periods. For instance, in lifting and transporting extremely large loads where extra weight capacity may be required, it may be advantageous to temporarily increase the output current of the generator to provide a boost in power to the electromagnet. However, should the power boost require an increase in current above the current rating of the fuse, the operator may risk blowing the fuse and disabling the machine. Thus, in order to provide overload current protection, without unnecessarily disabling the machine, an overload protection system capable of discriminating between electrical faults and temporary, deliberate current peaks may be required.
The presently disclosed overload protection system for electromagnetic lifts is directed toward overcoming one or more of the problems set forth above.